Method and structure for forming MOSFET with reduced parasitic capacitance

ABSTRACT

A method (and structure) of fabricating an MOSFET (metal-oxide-semiconductor field-effect transistor), includes, on a gate structure coated with a high-k sidewall spacer film, etching off the high-k sidewall spacer film from a top surface of the gate structure and from a portion of vertical walls of the gate structure. The etched-off high-k sidewall spacer film on the vertical walls is replaced with an ultra low-k material.

This Application is a Continuation Application of U.S. patentapplication Ser. No. 15/196,591, filed on Jun. 29, 2016.

BACKGROUND

The present invention relates to MOSFET devices, and more specifically,the formation of an ultra low-k film between the gate and source/draincontacts reduces the gate-source and gate-drain parasitic capacitances.

FIG. 1 shows exemplarily a finFET (Fin Field Effect Transistor) 100, atype of non-planar transistor used in many modern processor designs. Itcan be fabricated on an SOI (silicon on insulator) substrate or on a Si(silicon) substrate and is characterized by one or more fin structures102 that form the conductive channel controlled by the gate structure104. This fin-shaped structure permits multiple gates to operate on asingle transistor, such as demonstrated by structure 110, and permitsdevices that are smaller, faster, and more energy efficient.

The present inventors have recognized that the shrinking of the finFETstructure results in an undesired relatively high parasitic capacitancebetween the gate and source/drain contacts and have identified variousfactors in the conventional fabrication of finFET devices thatcontribute to this high parasitic capacitance, as follows. First, theshrinking of the gate pitch limits the spacer thickness, and a thinnerspacer provides a capacitor structure with higher capacitance.Additionally, an etchant that is selective to the material used for thespacer limits the options for the spacer material. Finally, the spaceris often damaged during the contact open stage of fabrication, which isthe fabrication stage during which the source and drain regions areexposed for metal deposition for contacts.

The present invention discloses a novel flow and unique structure toresolve the above-identified issues. Although the following discussionuses the finFET for purpose of explanation, the present invention is notintended as limited specifically to finFET structures since it isequally applicable to any MOSFET-like structure having a gate structurewith spacers to separate the gate from the source/drain structures.

SUMMARY

FIG. 2 shows an exemplary conventional finFET structure 200, from a planview 202 and from a cross-sectional view 204, after the post gate cappolish (CMP). As shown in the cross-sectional view 204, the conventionalstructure includes SiBCN (Silicon-Boron-Carbon-Nitride) thin film gatespacers 206, which material has a high-k characteristic. The presentinventors have recognized that such high-k material inherently resultsin a higher parasitic capacitance between the gate structure 208 andsource/drain structures 210, 212 (source/drain epitaxial regions) thanwould result if the spacer material had different characteristics.However, SiBCN is conventionally used because its thermalcharacteristics support the initial formation of the gate structures.

Accordingly, the present invention teaches to modify the conventionalgate spacers 206 having high-k composition to at least partially replacethis film with a material having an ultra low-k characteristic. Suchmodification decreases the dielectric characteristic so that thismodified gate spacer can no longer serve as efficiently as a capacitor,thereby decreasing its parasitic capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates exemplary conventional finFET structure 100;

FIG. 2 shows additional details 200 of the conventional finFET structurefrom a plan view 202 and cross-sectional view 204, for purpose ofexplaining the present invention;

FIG. 3 illustrates a cross-sectional view 300 of the source/draincontact open mask stage of fabricating a finFET in accordance withconcepts of the present invention;

FIG. 4 illustrates a cross-sectional view 400 after an isotropic etch toremove portions of SiBCN gate sidewalls;

FIG. 5 illustrates a cross-sectional view 500 after deposition of theultra low-k film;

FIG. 6 illustrates a cross-sectional view 600 after a vertical etch 602of the ultra low-k film;

FIG. 7 illustrates a cross-sectional view 700 after tungsten metaldeposition 702;

FIG. 8 illustrates a cross-sectional view 800 of a second exemplaryembodiment in which the SiBCN spacer 802 is etched only to the level ofthe surface of the source/drain epi layer 804;

FIG. 9 illustrates a cross-sectional view 900 after deposition of theultra low-k film 902 in the second embodiment;

FIG. 10 illustrates a cross-sectional view 1000 after a vertical etch ofthe ultra low-k film 902 in the second embodiment;

FIG. 11 illustrates a cross-sectional view 1100 after a source/draincontact metal deposition;

FIG. 12 shows in flowchart format 1200 the method of the invention; and

FIG. 13 shows a MOSFET configuration corresponding to an exemplaryembodiment described involving finFETs.

DETAILED DESCRIPTION

With reference now beginning with FIG. 3, two exemplary embodiments willnow be explained for finFETs. As mentioned above, the present inventionis also applicable in other devices, such as MOS-like devices, havingspacer elements separating a gate structure from the source/drain. FIG.3 shows a cross-sectional view 300 and plan view 302 at the stage thatthe oxide layer 214 of conventional device shown in FIG. 2 has beenetched in preparation for the the source/drain contact open mask 304,using, for example, an oxide RIE (reactive ion etch).

A first exemplary embodiment will be explained beginning with FIG. 4. Acharacteristic feature of this first exemplary embodiment is that a topof a residual of the SiBCN spacer 402 extends below the top of epi layer404. As shown in FIG. 4, the first step of this first embodiment is anetch of the exposed SiBCN spacer (306 in FIG. 3), so as to create adivot 406 (e.g., a cavity or space) between the gate and source/drain atthe epi layer 404, by over-etch. The etch of the SiBCN can beimplemented either as an isotropic etch or as an RIE (reactive ionetch). A portion of the SiBCN layer remains above the fin in order toprotect the gate structure during the downstream processes describedshortly by inadvertently permitting the fin to contact either the gatedielectric or metal gate.

The divot 406 is a high-aspect-ratio structure. Therefore, during spacermaterial deposition using an ultra low-k material and as exemplarilyshown in FIG. 5, it is easy to pinch off the divot on its top area totrap an air gap inside the divot space. This pinch-off characteristic,with its associated air pocket remaining inside the divot cavity, isintentional in the first exemplary embodiment, since it avoids having touse additional steps to completely fill in the etched-out space, whileproviding an ultra low-k value for this vacated region between the gateand the source/drain epi regions, since air also provides an ultra low-kvalue close to 1.

FIG. 5 shows the structure 500 after the ultra low-k film 502deposition, including air spacer 504 resultant from the divot pinch-offeffect. Unlike the original SiBCN spacer (k-value is around 5), ultralow-k material, for example SiCOH (a thin film comprising silicon Si,carbon C, oxygen O, and hydrogen H and having a k-value of 2), isdeposited as a partial substitute spacer material 502. Such ultra low-kmaterials can be used because there is no more high thermal budget(e.g., source/drain dopant and activation anneal) required for the BEOL(Back End of the Line) processing of the devices, nor is any moreaggressive cleaning (epi preclean) needed.

The first embodiment is characterized by air spacers 504 between the epilayer and the gate, and the combination of the air space with k≈1 andSiCOH with k≈2 provides an ultra low-k gate spacer that reduces thegate/source/drain parasitic capacitance. Other possible (non-limiting)examples of ultra low-k materials would be organosilica glasses (OSG),porous xerogel, or mesoporous silica films (MCM). In FIG. 6, an RIE(reactive ion etch) 602 provides a vertical etch to remove the topsurface of the ultra low-k film while retaining the vertical componentsof the film on the sidewalls of the gates.

In FIG. 7, tungsten 702 is deposited for the source/drain metal, to befollowed by routine planarization, resulting in a finFET structurehaving ultra low-k spacers 704 with air spacers, as well as portions 706of the original SiBCN spacer.

FIG. 8 shows an exemplary second embodiment 800, which differs from thefirst exemplary embodiment 700 by reason that the SiBCN spacer 802 isnot over-etched as in the first embodiment. Thus, in the secondembodiment the original underlying SiBCN spacer film is retained to beat the same height as the source/drain epi 804 top surface. As shown inFIG. 8, again, an etch (either isotropic or RIE) is used to remove theoriginal SiBCN spacer layer (see 306, FIG. 3) from the gate structure.

Typically, the exposed SiBCN can first be removed rapidly, which wasfollowed in the first embodiment by an additional etch to additionallyremove the SiBCN spacer between the gate and epi layer. Thus, the etchof the original SiBCN spacer film 306 is faster in the second embodimentbecause no additional etch time is required to over-etch SiBCN filmmaterial below the top of the source/drain epi regions. As in the firstembodiment, a portion of SiBCN again remains above the fin so that thedownstream processes will not damage the gate stack.

FIG. 9 shows the fabrication structure 900 of the second embodimentafter the ultra low-k film 902 deposition, and, similar to exemplaryembodiment 1, the ultra low-k material 902 can be used because there isno more high thermal budget required. FIG. 10 shows the vertical etch(e.g., RIE 1002) used to remove tops of the ultra low-k film 902 fromthe top of the gate structures and the source/drain epi areas, leavingultra low-k film as the vertical sidewall spacers of the gates. FIG. 11shows the tungsten metal deposition 1102 for the source/drain contactsand further planarization, resulting in the finFET structure 1100 withultra low-k spacers 1104 and portions 1106 of the original SiBCN spacermaterial.

In comparing FIG. 7 with FIG. 11, it should be clear that the exemplarysecond embodiment does not include the air spacer present in the firstexemplary embodiment.

In the first exemplary embodiment, the gate spacer comprises the ultralow-k spacer as a top portion, the air gap as a middle portion, and theoriginal SiBCN layer as a bottom portion. In the second exemplaryembodiment, the spacer comprises the ultra low-k spacer as a top portionand the original SiBCN spacer layer as a bottom portion. Therefore,because of the low-k effect of the air spacer, the first exemplaryembodiment has an advantage of providing a lower parasitic capacitancethan that of the second embodiment. However, the first embodiment hasthe disadvantage that the divot height is not easy to control precisely.

FIG. 12 shows in flowchart format 1200 processing steps related to thetwo exemplary embodiments. In step 1202, the device is fabricated in theconventional manner for the Front-End-of-Line (FEOL) and Middle-of-Line(MOL) processings, meaning that the fabrication stages for fabricatingthe pattern of components in the substrate uses conventional FEOLprocessing and the gate structure fabrication uses conventional MOLprocessing, including SiBCN material for gate spacers. In step 1204 theconventional S/D Open Mask step etches the oxide layer. In step 1206,the conventional SiBCN spacer is etched partially from the verticalwalls of the gate structures, leaving a lower portion to protect againstsubsequent damage at the gate/channel interface. In step 1208, the ultralow-k material is formed on the vertical gate structure walls, and, instep 1210, the source/drain contacts are completed.

FIG. 13 shows a planar MOSFET configuration that exhibits an exemplaryembodiment described above, having source/drain 1302, 1304 and an upperportion of ultra low-k spacer material 1306. A portion 1308 remains ofthe original SiBCN spacer, to protect against damage at the gate/channelinterface.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of fabricating an MOSFET(metal-oxide-semiconductor field-effect transistor), said methodcomprising: on a gate structure coated with a high-k sidewall spacerfilm, etching off said high-k sidewall spacer film from a top surface ofsaid gate structure and from a portion of vertical walls of said gatestructure; depositing an ultra low-k material as a replacement for saidetched-off high-k sidewall spacer film on said top surface and saidvertical walls such that a thickness of said ultra low-k material issubstantially constant on said top surface and on said vertical walls;and etching away, using a vertical etch, said ultra low-k material onlyon said top surface of said gate structure, thereby retaining said ultralow-k material on said vertical walls with said substantially constantthickness, wherein said gate structure controls carrier movement in anunderlying channel below said gate structure and said gate structure isadjacent on each of two sides to epitaxial regions serving assource/drain functions for said MOSFET, wherein said etching off of saidhigh-k sidewall spacer film comprises an etching of an exposed portionof said high-k sidewall spacer film followed by an over-etching of saidhigh-k sidewall spacer film below a top surface of said epitaxialregions, thereby creating high-aspect-ratio divots adjacent to said gatestructures, said high-aspect-ratio predetermined to permit a pinchingoff by a deposition of said ultra low-k material that will permit airspacers to form in said divots, said air spacers providing an ultralow-k value inside regions in said portion of vertical walls that arevacated by said over-etching.
 2. The method of claim 1, furthercomprising depositing said ultra low-k material, said depositingpinching off said divots to create said air spacers.
 3. The method ofclaim 1, wherein said etching off of said high-k sidewall spacer filmcomprises an etching that stops etching off said high-k sidewall spacerfilm at a top surface of said epitaxial regions, so that said replacingsaid etched-off high-k sidewall spacer film with said ultra low-kmaterial comprises depositing said ultra low-k material on sidewalls ofsaid gate structure above said top surface of said epitaxial regionssuch that said sidewalls of said gate structure are covered by a spacerfilm comprising a bottom portion of said high-k sidewall spacer filmbelow said top surface of said epitaxial regions and a top portion of alayer of said ultra low-k material above said top surface of saidepitaxial regions.
 4. The method of claim 1, wherein said MOSFETcomprises a finFET.
 5. The method of claim 1, wherein said high-ksidewall spacer film comprises a SiBCN (Silicon-Boron-Carbon-Nitride)thin film.
 6. The method of claim 1, wherein said ultra low-k materialcomprises at least one of: SiCOH, a thin film comprising silicon Si,carbon C, oxygen O, and hydrogen H; an organosilica glass (OSG); aporous xerogel; and a mesoporous silica films (MCM).
 7. The method ofclaim 1, wherein said high-k sidewall spacer film has a k-value ofapproximately 5 and said ultra low-k material has a k-value no more thanapproximately
 2. 8. A MOSFET fabricated by the method of claim
 1. 9. Themethod of claim 2, wherein said etching off said high-k sidewall spacerfilm from said portion of vertical walls of said gate structure stops toretain a bottom portion of said high-k sidewall spacer film such thatsaid sidewalls of said gate structure are covered by a spacer filmcomprising a bottom portion of said high-k sidewall spacer film, amiddle portion of said air spacers below a top surface of said epitaxialregions, and a top portion of a layer of said ultra low-k material. 10.A method of fabricating an semiconductor device, said method comprising:on a gate structure coated with a high-k sidewall spacer film, etchingoff said high-k sidewall spacer film from a portion of vertical walls ofsaid gate structure; and depositing an ultra low-k material as areplacement for said etched-off high-k sidewall spacer film on said topsurface and said vertical walls; and etching away, using a verticaletch, said ultra low-k material only on said top surface of said gatestructure, thereby retaining said ultra low-k material on said verticalwalls, wherein said gate structure controls carrier movement in anunderlying channel below said gate structure and said gate structure isadjacent on each of two sides to epitaxial regions serving assource/drain functions for said semiconductor device, and wherein saidetching off of said high-k sidewall spacer film comprises an etching ofan exposed portion of said high-k sidewall spacer film followed by anover-etching of said high-k sidewall spacer film below a top surface ofsaid epitaxial regions, thereby creating high-aspect-ratio divotsbetween said epitaxial regions and said gate structures, saidhigh-aspect-ratio predetermined to permit a pinching off by a depositionof said ultra low-k material that will permit air spacers to form insaid divots when said ultra low-k material is deposited.
 11. The methodof claim 10, further comprising depositing said ultra low-k material,said depositing pinching off said divots to create said air spacers. 12.The method of claim 11, wherein said etching off said high-k sidewallspacer film from said portion of vertical walls of said gate structurestops to retain a bottom portion of said high-k sidewall spacer filmsuch that said sidewalls of said gate structure are covered by a spacerfilm comprising a bottom portion of said high-k sidewall spacer film, amiddle portion of said air spacers below a top surface of said epitaxialregions, and a top portion of a layer of said ultra low-k material. 13.The method of claim 10, wherein said high-k sidewall spacer filmcomprises a SiBCN (Silicon-Boron-Carbon-Nitride) thin film, aspredetermined to have thermal characteristics to support an initialformation of said gate structure.
 14. The method of claim 10, whereinsaid ultra low-k material comprises at least one of: SiCOH, a thin filmcomprising silicon Si, carbon C, oxygen O, and hydrogen H; anorganosilica glass (OSG); a porous xerogel; and a mesoporous silicafilms (MCM).